The present invention relates to an electrically weldable sleeve for joining thermoplastic pipeline parts, which has a tubular sleeve member made from a thermoplastic material, an electrical heating element embedded into the inner wall of the sleeve member and terminals for the supply of electrical energy to the heating element. The electrical energy supplied to the electric heating element is used for producing heat for the purpose of forming a melting zone on welding the sleeve member to the portions of the pipeline parts located in the melting zone.
Electrically weldable sleeves made from the same or a similar thermoplastic material are known for joining thermoplastic pipeline parts. The term pipeline parts is understood to cover straight and curved pipeline sections, shaped pipe portions and pipeline fittings. For the purpose of joining to other pipeline parts, said pipeline parts have tubular connection pieces, on to which is placed a sleeve of the aforementioned type and is electrically welded therewith to form a drip-proof and gas tight joint. Electrically weldable sleeves of the aforementioned type are known in numerous different constructions, reference being made e.g. to U.S. Pat. Nos. 3,943,334 and 4,117,311.
If such known electrically weldable sleeves are used for joining pipeline parts, the attaining of a reliable, tight connection is dependent on different characteristics of the sleeve. A particularly important characteristic of a weldable sleeve is its capacity to shrink by releasing latent stresses during its heating occurring during the welding process, in order to remove the clearance between the weldable sleeve and the connection pieces of the pipeline parts to be welded. For this purpose it is known to impart a shrinkage reserve to the thermoplastic sleeve member during the production of the sleeve. This is released during the heating taking place during the welding process for eliminating the existing clearance between the sleeve and the connecting pieces of the pipeline parts.
Simultaneously with the release of the shrinkage reserve and the removal of the clearance between the sleeve and the connection piece of the pipeline part, a volume contraction occurs during the cooling of the welded joint in the welded zone when the molten material passes into the solid state. This volume contraction can be considerable and is e.g. approximately 20% in the case of polyolefinic materials. If the shrinkage reserve imparted to the sleeve member is not sufficient, to completely compensate for the volume shrinkage through the shape change occurring during shrinking, cavities form in the solidifying melting zone. Since as a result of the heat gradient occurring on the sleeve member, cooling advances from the edges of the melting zone towards the center thereof and there is an increase in the proportion of the sleeve member which is supported on the connecting pieces projecting into the sleeve. This supporting action of the connecting pieces acting counter to the shrinkage of the sleeve means that in the final area of the not yet solidified melting zone, the sleeve member completely follows the volume shrinkage occurring, and can compensate same by a shape change. Thus, cavities impairing the strength of the sleeve occur in the center of the melting zone. Such cavities can in particular occur in the case of relatively thick-walled sleeve members, such as conventionally occur when joining pipeline parts which are to be operated under pressure.